I was working on an older Murray rider yesterday (11hp Briggs) and I just couldn't get it to idle smoothly if it would idle at all. After the Sunday family stuff was done I crept back out to the garage and pushed it outside for another go at it.
It was dark, and while I had it running I could see a spark behind the carb. Turns out the throttle rod had rubbed through the coil kill wire insulation and was creating an intermittent miss when the throttle rod was moved to the idle position. I pushed it out of the way and off she went smooth as can be!
Always check the simple stuff when things just aren't going your way. :biggrin: